1. Field of the Invention
This invention relates generally to a collector optic for a light source and, more particularly, to a collector optic for a laser plasma extreme ultraviolet (EUV) radiation source, where the collector optic includes an assembly of silicon parts fused together by a glass frit bonding process and where the collector optic includes a liquid cooling system.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current trend for photolithography light sources is to develop a system that generates light in the extreme ultraviolet (EUV) or soft X-ray wavelengths.
Various devices are the known in the art to generate EUV radiation. One of the most popular EUV radiation sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Argon and Krypton, and combinations of gases, are also known for the laser target material. In the known EUV radiation sources based on laser produced plasmas (LPP), the gas is typically cryogenically cooled in a nozzle to a liquid state, and then forced through an orifice or other nozzle opening into a vacuum process chamber as a continuous liquid stream or filament. The liquid target material rapidly freezes in the vacuum environment to become a frozen target stream. Cryogenically cooled target materials, which are gases at room temperature, are required because they do not condense on the source optics, and because they produce minimal by-products that have to be evacuated by the process chamber. In some designs, the nozzle is agitated so that the target material is emitted from the nozzle as a stream of liquid droplets having a certain diameter (30-100 xcexcm) and a predetermined droplet spacing.
The target stream is radiated by high-power laser beam pulses, typically from an Nd:YAG laser, that heat the target material to produce a high temperature plasma which emits the EUV radiation. The frequency of the laser beam pulses is application specific and depends on a variety of factors. The laser beam pulses must have a certain intensity at the target area in order to provide enough energy to generate the plasma. Typical pulse durations are 5-30 ns, and a typical pulse intensity is in the range of 5xc3x971010-5xc3x971012 W/cm2.
FIG. 1 is a plan view of an EUV radiation source 10 of the type discussed above including a nozzle 12 having a target material storage chamber 14 that stores a suitable target material, such as Xenon, under pressure. A heat exchanger or condenser is provided in the chamber 14 that cryogenically cools the target material to a liquid state. The liquid target material is forced through a narrowed throat portion or capillary tube 16 of the nozzle 12 to be emitted under pressure as a filament or stream 18 into a vacuum process chamber 26 towards a target area 20. The liquid target material will quickly freeze in the vacuum environment to form a solid filament of the target material as it propagates towards the target area 20. The vacuum environment in combination with the vapor pressure of the target material will cause the frozen target material to eventually break up into frozen target fragments depending on the distance that the stream 18 travels and other factors.
A laser beam 22 from a laser source 24 is directed towards the target area 20 in the process chamber 26 to vaporize the target material. The heat from the laser beam 22 causes the target material to generate a plasma 30 that radiates EUV radiation 32. The EUV radiation 32 is collected by collector optics 34 and is directed to the photolithography apparatus (not shown), or other system using the EUV radiation 32. The collector optics 34 is an ellipsoidal reflector dish, where the target area 20 is at the near focal point of the optics 34, and where the aperture for the photolithography apparatus is positioned at the far focal point of the collector optics 34. In this design, the laser beam 22 propagates through an opening 36 in the collector optics 34, as shown.
The collector optics 34 is designed to have the desired collection efficiency, have a minimum angle of incidence of the EUV radiation 32 on the collector optics 34, and have the proper maximum angle of the radiation 32 at the far focal point so that as much of the EUV radiation 32 is collected by the photolithography apparatus.
It is desirable that as much of the EUV radiation 32 as possible be collected to improve source efficiency. For example, the higher the intensity of the EUV radiation 32 for a particular photolithography process, the less time is necessary to properly expose the various photoresists and the like that are being patterned. By decreasing the exposure time, more circuits can be fabricated, thus increasing the throughput efficiency and decreasing the cost. Further, by providing more useable EUV radiation from the collector optics 34, the intensity of the laser beam 22 can be lower, also conserving system resources.
Optimizing the reflectance of the reflective surface of the collector optics 34 is one way in which the amount of the EUV radiation 32 that is collected can be increased. Typically, the reflective surface of the collector optics 34 is coated with a reflective coating to enhance its reflectance. However, it is also important that the coating material not contaminate source components in response to the high energy ions generated by the plasma 30 that may impinge the reflective surface and release coating material. One such coating that provides the desired characteristics is a silicon/molybdenum (Si/Mo) multilayer coating. However, the best Si/Mo multilayer coating on the collector optics 34 only reflects about 70% of the EUV radiation 32 impinging thereon, even at its theoretical maximum performance.
The incident radiation 32 generated by the LPP that is not reflected by the collector optics 34 is absorbed by the collector optics 34. The absorbed radiation heats the collector optics 34 to such a degree that the Si/Mo multilayer coating on the optics will be damaged unless suitable cooling is provided. Additionally, heating the collector optics 34 causes it to expand that changes the dimensions of the optics 34 affecting its optimal performance. Therefore, suitable design considerations must be provided to insure that the collector optics 34 is adequately cooled and configured so that the heating does not adversely affect performance of the source.
In accordance with the teachings of the present invention, a collector optic assembly for an EUV radiation source is disclosed. The collector optic assembly includes an elliptically shaped meniscus having a reflective Si/Mo coating for collecting and reflecting EUV radiation generated by the source. The meniscus is machined from a single piece of silicon. The collector optic assembly further includes a heat exchanger that includes cooling channels through which flows a liquid coolant. The heat exchanger is fabricated from a plurality of silicon sections fused together by a glass frit bonding process. The meniscus is fused to a front side of the heat exchanger by a glass frit bonding process. A liquid coolant inlet manifold and a liquid coolant outlet manifold are also each machined from a single silicon block and are mounted to a back side of the heat exchanger. Inlet coolant pipes are mounted to the inlet manifold, and outlet coolant pipes are mounted to the outlet manifold so that a liquid coolant can be introduced into the cooling channels through the inlet pipes and the inlet manifold and the warm coolant can be output from the heat exchanger through the outlet manifold and the outlet pipes.
Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.